Spectrum is a limited and precious resource for wireless communications. Currently all the golden spectrum have been allocated and utilized. However, demand for extra capacity seems endless. According to statistics, only 20% of the licensed spectrum is in use in any moment.
Software Defined Radio (SDR) is a concept to build a radio in software form so that one design can serve multiple standards, multiple air interfaces. With a software defined radio, most of the parameters, such as modulation schemes and coding schemes, bandwidth or sampling rate, filtering etc can be changed in software. Therefore software defined radio needs a very high sampling rate analogue-to-digital (ADC) to handle a variety of bandwidth from very narrow band (GSM 200 kHz) to very wide band (WiMax 20 MHz). SDR also needs a very powerful processor to handle from very low data rate (SMS, speech) to very high data rate (100 Mbps). Since its first inception in 1995 (first demo for a military project) [1], SDR has been one of the major research and development areas. FCC has adopted changes to the equipment authorization rules to accommodate SDR [2]. However, after more than 10 years of research and development, the perception on SDR is still perceived to be worthwhile.
3rd Generation Partnership Project (3GPP) is a collaboration agreement (established in 1998) among many countries and is the organization responsible for building a globally applicable 3rd generation mobile wireless communication system. A system based upon this 3rd generation system is also known as a Universal Mobile Telecommunication System (UMTS).
The UMTS system uses wideband CDMA (code division multiplexing access) technology. The important notion for CDMA is spectrum spreading (refer to FIG. 1). The real information spectrum 10 is spread via a OVSF (orthogonal variable spreading factor) code to produce a spread spectrum signal 14. The OVSF code 12 is shown in detail in FIG. 2.
Referring to FIG. 3, there is illustrated three major channels for the 3GPP/UMTS downlink. The first are the synchronization channels 20 including a primary synchronization channel (PSCH) 22 and a secondary synchronization channel (SSCH) 24. The second is a pilot channel 26 and the third is a data channel 28.
The primary synchronization channel (PSCH) 22 and secondary synchronization channel 24 are used by a terminal to determine the BTS timing information and scrambling code information so that it can access the system.
The pilot channel 26 allows the terminal to do finger detection, channel estimation and SNR estimation to decode information.
Data channels 28 are used to carry user information such as voice, packet/circuit data. These channels are superposed to form a 3GPP/UMTS signal before transmission as shown in FIG. 4.
The PSCH/SSCH have only 256 chips in each slot and the rest of each slot are filled by zeros. The evolution of 3GPP/UMTS, such as HSDPA, HSOPA and LTE is still following this basic 10 ms frame structure to maintain compatibility.
WiMax System and Frame Structure
WiMax system is defined by IEEE 802.16x standards and uses Orthogonal Frequency Division Multiplexing (OFDM) technology. A typical OFDM transmitter and receiver are illustrated respectively in FIG. 5 and FIG. 6.
Referring to FIG. 5, there is illustrated a transmitter 40 with one antenna 42. Information bits are fed into a forward error correction (FEC) encoder 44 that encodes the bits with some redundancy built in. The coded bits are then mapped 46 to constellation symbols. After a serial to parallel conversion 48, an inverse fast Fourier transform (IFFT) 50 with appropriate size (say 64 for WiFi, 1024 for WiMax) is applied. The output of IFFT 50 is then converted back to serial format 52 and a cyclic prefix (CP) is usually appended in front 54 so that the linear convolution can be automatically translated into a cyclic convolution after removing it in the receiving end. A windowing filter 56 is applied for the data blocks to control the adjacent emission masks to meet the specification requirement. A digital to analogue converter (DAC) 58 up converts the signal to analog format, which is amplified (not shown) and radiated from the antenna 42.
Referring to FIG. 6, there is illustrated a receiver 60 with one antenna 62. In the receiving end, the received signal is down converted and digitized by an analogue to digital converter (ADC) 64. A matching filter 66 can be applied to maximize the signal gain. The digitized data is fragmented accordingly and the portion of CP is removed 68. The fragmented data is then converted to a format 70 suitable for fast Fourier transform (FFT) 72. After FFT 72, the multipath channel is estimated 74 via known sequences (such as pilots, training sequence, preamble etc) and a maximum likelihood detection is usually applied to map the received data back to constellation symbol level 76 either in hard bit or soft bits, which are then input to a FEC decoder 78.
For a 5 MHz spectrum for example, the Nyquist sampling rate specified in the standard (Refer to [6]) is 5.6 MHz. In order to meet the spectrum mask, some guard tones (also commonly called subcarriers) are used when designing an OFDM symbol. The guard tones are not modulated therefore the transmitted OFDM signal spectrum power density is concentrated within the middle part of the RF channel. An example OFDM symbol design of WiMax with 5 MHz is illustrated in FIG. 7.
First, the 5.6 MHz spectrum 80 is divided into 512 pieces (also called subcarriers or tones) of 10.9315 kHz each, which is usually referred to as subcarrier spacing. The left-hand side 46 pieces 82 and right-hand side 45 pieces 84 are not used for transmission. These non-used pieces add up to 91*10.9315 kHz or roughly 0.995 MHz. Therefore only 5.6-0.995=4.6052 MHz 86 are actually being used. This may be adapted to fit within a 5 MHz band.
Similarly, an example for 10 MHz bandwidth is illustrated in FIG. 8. The 11.2 MHz spectrum 88 is divided into 1024 pieces (also called subcarriers or tones) of 10.9315 kHz each. The left-hand side 92 pieces 90 and right-hand side 91 pieces 92 are not used for transmission. The total spectrum being used 94 is 9.1995 MHz. This may be adapted to fit within a 10 MHz band.
From the OFDM symbols design illustrated in FIGS. 7 and 8, we can see that OFDM system uses guard tones to shape its spectrum to restrict the adjacent channel interference. This is quite different from conventional TDMA and CDMA systems where they use shaping filters such as Gaussian filter or RRC (Root Raised Cosine) to shape the spectrum within the specified masks.
An example of WiMax/IEEE 802.16e TDD frame structures is illustrated in FIG. 9. In this example, every 5 ms is divided into four parts    (1) DL (downlink) 95 part in which the BTS transmits and terminal receives    (2) TTG: Transmit/receive transmission gap 96.    (3) UL (uplink) part 97 in which terminal transmits and BTS receives    (4) RTG: Receive/transmit transition gap 98.